Plasma processes are frequently used to modify or treat the surfaces of workpieces such as semiconductor wafers, flat-panel display substrates, and lithography masks. Conditions within a plasma process are designed to produce a complex mixture of ions, reactive chemical species (free radicals), and energetic neutral species. The interaction of these materials then produces the desired effect on the surfaces of workpieces. For example, plasma processes are used to etch materials from the surfaces of semiconductor wafers so as to form complex electrical elements and circuits. The conditions within the plasma process are carefully controlled to produce the desired etch directionality and selectivity.
The surface modifications produced by a specific plasma are sensitive to a number of basic parameters within the plasma. These parameters include such variables as: chemical concentrations (partial pressures), temperatures (both surface and gas phase), and electrical parameters (ion fluxes, ion energy distribution functions). A number of these parameters (e.g. gas concentrations and pressure) can generally be easily controlled using external actuators such as Mass Flow Controllers (MFCs) and servo driven throttle valves. Other important parameters (e.g. temperatures and free radical concentrations) can often be observed or measured via sensor systems (e.g. thermocouples and Optical Emission Spectrometers (OES)) installed on the process tool. A last set of important parameters such as ion fluxes and ion energies are more difficult to either directly control or monitor.
US publication No. 2005-0151544 discloses a plasma processing system with diagnostic apparatus for making in-situ measurements of plasma properties. The diagnostic apparatus generally comprises a non-invasive sensor array disposed within a plasma processing chamber, an electrical circuit for stimulating the sensors, and means for recording and communicating sensor measurements for monitoring or control of the plasma process. In one form, the sensors are dynamically pulsed dual floating Langmuir probes that measure I-V characteristic, displacement RF current into or through the wafer and self-bias due to electrons piling up on the surface, which can be used to determined the charge on the wafer.
Wafer charges are formed due to different flux rates for ions and electrons (due to their very different masses). Wafer charging can lead to damage to the devices. One type of tool that is conventionally used for characterizing wafer charging during wafer processing in ion-based and plasma-based IC processing equipment includes EEPROM-based peak potential sensors and current sensors to characterize the I-V relationship of charging transients. The gate of the transistors is coupled to the antenna structures on the wafer. The device measures the cumulative charge, not charge as a function of time. Furthermore, the wafer has to be taken out of the plasma chamber to read the charge measurement.
US publication No. 2006-0249729 discloses a sensor wafer that uses a triple capacitor stack to measure apparent alternating current (AC) at the surface of the wafer. This rectification (detection) device has a minimum bias requirement and a strong frequency dependency on the range of interest. The measurement is purely AC and the center capacitor, formed by a polyimide substrate is the shunt impedance that generates the AC potential to be measured. The sensor responds in a confounded way to a number of electrical parameters in the plasma chamber and is unable to relate specifically to any one parameter. This makes it difficult to find the right “knobs” to tune the chamber when problems are encountered.
In addition, many prior art sensor wafers include a module atop of the wafer that houses electronics for the sensor array. This module can cause severe disturbance in the plasma or can be a point of discharge damages and can also be a source for contamination.
Another problem with prior art sensor wafers is that sensor pads in the array and electrical connections between these pads and associated electronics are often made of metal traces, e.g., Aluminum, that is deposited on the surface of the wafer. Exposure to plasma, e.g., Argon plasma, eventually erodes aluminum traces on the surface of the wafer. In some sensor wafers, entire surface of the wafer is covered by polyimide to protect the traces and sensor pads. However, the polyimide coating can have a very short life time in certain plasma environments and may also be a source of contamination. In addition, the use of certain metals, such as copper, is strongly avoided in many process steps.
It is within this context that embodiments of the present invention arise.